My first reaction was, "No way." Then I looked into this more, and now I think the ‘scope may have indeed been the culprit.

The telescope is a design called a Schmidt-Cassegrain, or just SCT. I have one myself! It’s a tube with a big mirror (called the primary) in the back that’s curved. It reflects light back up the tube. At the top of the tube is a flat piece of glass (called a corrector plate) with a smaller mirror embedded in the middle. This reflects the light again down toward the bottom of tube, where it passes through the hole in the big mirror and into an eyepiece (and from there into your eye or camera). The inset diagram here shows how this works; click to newtonianate.

I called the Carefree fire department, and talked to Colin Williams, their Press Information Officer. He was very helpful, and told me more of what happened. The place where the fire was most intense — and therefore the likely origin — was on the ceiling, a few feet to the side of being directly above the telescope. Also, a few days ago a neighbor had noticed that there was an intense spot of light on the ceiling coming from the telescope. At the time, the owner didn’t think much of it.

I’m thinking that the telescope may have been pointed near the horizon. The setting Sun happened to pass into the line of sight of the ‘scope — perhaps he was setting up to see the Transit of Venus. Most people who use an SCT have a device called a diagonal which reflects light at a 90° angle, making it easier to use (you don’t have to stoop over as far to see through the eyepiece). For a horizontal telescope, this means the light coming out the back end could’ve been projected up onto the ceiling.

Here’s a shot of the burned out ‘scope:

You’re seeing the top of the telescope there on the left. The corrector plate is broken, most likely due to falling debris. The telescope is shown as it was found, but you can’t say for sure that’s the position it was in when the fire started; note the debris is essentially on top of it. The telescope could’ve started off more horizontal and been whacked by something falling, knocking it into the position we see it here.

It does seem unlikely that the ‘scope happened to be pointing at the Sun, but weirder things have happened. It’s also possible that the primary (big) mirror was misaligned, and sent focused light straight up and out of the tube at a slight angle, causing the fire. That seems even less likely to me, though. Had the mirror been misaligned it would’ve been impossible to use, and the geometry is all wrong for that.

So I think what we have here is a likely scenario — the telescope dunnit! — but the details are unclear.

The lesson here? Telescopes are powerful instruments. When you’re done observing, put away all the accessories and stow it in a safe place and position. And put the protective cover on it! At the very least that keeps the corrector plate protected from dust and scratches. And in this case, it may have prevented a fire, too.

Comments (70)

JHGRedekop

Back when I was in university, I made the mistake of decorating my dorm window with hanging Fresnel lenses. A week or so later, I noticed scorched lines on the drapes where the sunlight had been focused for a few minutes each morning. So the lenses came down.

I saw an 18″ Dbosonian at a star party in Canada start to smolder as the Sun started to shine down the shrouded truss tube. The front of the scope had been left uncovered, and the almost focussed light that bounced off the mirror was hot enough to start the cloth shroud smoking.

So…always point your scope to the North if you live in the northern hemisphere and south if you don’t. Covering the front of the scope when not in use is not a bad idea either!

A co-worker of mine once almost started a fire with an old style overhead projector. He was sitting on a table in the back of a conference room and accidentally kicked the power switch of the projector tucked under the table.

The Fresnel lens just happened to focus the light on the bottom of the table.

After a couple of minutes, we smelled burning wood and saw a small curl of smoke coming from between his legs.

Still seems very unlikely. Why, or even how, would the telescope focus to a point several feet beyond the location of the eyepiece? I tried to duplicate this with my SCT and couldn’t get anything like a tight enough spot to cause ignition, or even noticeably warm the wall surface.

It reminds me of one afternoon when I was a teenager, doing homework in my bedroom. I had a desk by the window, and on that window was a round mirror on a stand that I used for putting in my contact lenses in the morning. On the opposite side of the mirrow was a magnifying mirror (convex?)

Anyway, I was sitting there reading, and became aware of a smoky varnish smell. After looking around, I saw that the sunlight coming through the window was bouncing off the mirror and focused on a spot on the wooden window frame, where the wood varnish was smoking and bubbling. Yikes! I hate to think what might have happened if I’d had a stack of papers sitting there.

Had something similar to this happen many years back. My wife had one of those makeup mirrors sitting on her dresser. The sunlight reflecting off of the concave surface was focused back at the drapes and it burned a hole in them. Lucky that it didn’t start a fire.

I’m going to continue sceptical about this. mainly because it could only focus the light at the mirrors primary focus, which could only in two places: at the back end of the tube, where the eyepiece is, or, if the sun was off center, much of the light would miss the central secondary mirror, and form a focus the length of the tube in front. If there were drapes or similar maybe 15″ in front of the telescope, then these could very easily be set alight by the mirror.
But the eyepiece end could not. The focal point is within the tube. You can focus an image using an eyepiece – I know this because that is how I viewed the transit, albeit using a cheap newtonian – but the image created is large, and therefore the heat would not be concentrated. A 12 mm eyepiece focused a 5″ image on paper 12″ from the eyepiece. An image cast on the ceiling would be several feet across – or badly out of focus and even bigger.
Besides, with a ‘scope like that, the image of the sun would be 7mm (convert,.. convert,.. 5/16″?)across: Even with the size of the mirror, I doubt it would reach the kindling point of timber, or, if it did, not really enough to do more than char it.
So, yeah, fire investigator doesn’t know how a telescope works, news at 11.

BA, your article is not clear enough in the detail of exactly how this scope could plausibly have focussed sunlight into a sufficiently intense spot to start the fire.

For example, was it set up in such a way that it just happened to project a sharply-focussed image of the sun onto the ceiling? Even so, how did this get sufficiently intense to ignite whatever the ceiling was made of? After all, is this not how we are told to view an eclipse or transit?

If the telescope was pointed near the horizon, would the sunlight be intense enough (after passing through a substantial amount of atmosphere) to then start a fire?

Although the details, as you say, are not clear, it is also not clear (to me, at least) how the telescope might plausibly have caused the fire.

Robert — it is a common misconception that to start a house fire, you have to reach the ignition point of timber. It is this misconception that leads many smokers to believe they can’t accidentally start the house on fire with a dropped cigarette, because it’s too cool. This is untrue — many house fires are started by dropped cigarettes. Sometimes even by ones that the smoker believes were extinguished. (That happened in my hometown last year; a restaurant was severely damaged after a patron dumped a cigarette in a trash can. Staff dumped several pitchers of water into the can to put it out, but it was still smouldering, and eventually reignited many hours later. People heading to work on the nearby freeway the next morning noticed the smoke and called the fire department, who were able to save the structure although it was closed for over a year to clean up and renovate.) Fire can do very weird things sometimes, and casual experiments like trying to light paper with a Newtonian are probably not going to give you absolute answers to what would happen in this specific situation.

How long before telescope manufacturers have to put a “Warning: May burn your house down” label on their telescopes? Astronomy is a dangerous profession. 😀 I’m pretty skeptical of this, but I’m sure many people who have this type of telescope will try to see if it’s possible! Call MythBusters.

Also can’t wait for Phil’s views on the Dutch reality show which wants to send people to Mars in 10 years. I think the Gingrich Moon Base sounded more plausible.

Calli – you missed the point. We can’t get it to even slightly warm the surface that the image is projected on. The optical geometry is dead wrong for this telescope to have caused this fire. That some other object or some other reflecting surface can is irrelevant. Firemen are terrific, but they don’t generally understand optics well.

I’ve heard of mangnafying glasses starting fires I’ve done it myslef ,but a telescope? Mine stays inside untill night but I have had problems , When I frist moved in and used it ,several neighbors thought I was looking in there windos with it (NO I AM NOT A PEEPING TOM) even thro I had it pointed in the sky . After a while the neighbors became convinced I had no neferious pourpose with my telescope . May its the price we pay for exploreing the Universe.

Your scenario sounds plausible to me. I have a loose plastic cap over my diagonal, to keep dust out, and it’s certainly possible that something else performing the same duty could be ignited by a focused solar image.

Basically he was setting up his telescope to have a look at the morning sun. While he was preparing the sun filter he was distracted. He says he was looking away just for 20 seconds when his telescope started to smell funny and send some smoke signals.

A telescope can most certainly cause a fire. I am an amateur astronomer, and I accidentally left my refractor pointed in a direction which the sun’s path eventually crossed on its way to setting. The lens cap was still on the eyepiece, but the front end of the scope was open. The sun’s rays melted right through the eyepiece cover in a matter of seconds (and ruined the 200 dollar eyepiece with dripping melted rubber).

My friend’s car almost caught fire. We both have the same gear shift knob–it’s a dragon holding a glass orb. In his low-rider, the sun caught the orb and the seat was smoking when he noticed. In my truck, wrong angle, but we both use (matching) dragon sun shields now.

The sunlight doesn’t have to hit the secondary mirror to get focused – it is focused by the primary and if the angle of the sun is anywhere near the center, it will focus back out the front of the OTA.

Here in the mid-Atlantic area of the US, dew is a really bad problem with telescopes. In mornings after an all night run is a fairly common practice at star parties to point the scope in such a way so that the sun will shine on the front glass of an SCT, MCT, or a Refractor, or down the tube of a Newtonian so that the sun will heat up the glass and drive off the dew.

You typically point it at an oblique angle so that the sun doesn’t focus down the optical train. I’ve done this myself dozens of times with my 18″ f/4.5 truss dob.

At a star party several years ago a friend of mine did this with his 25″ f/5 truss dob. He wandered off and forgot about his scope for a couple of hours. In those several hours, the sun moved and what was an oblique angle was now almost dead on. The reflected and NOW FOCUSED sunlight was hitting a cedar tree about 10 feet to the side. Fortunately the tree was green and put off a lot of smoke and we caught it before it burst into open flames.

That’s a lot of variables to bring together for a SCT sitting in a house to start a fire. I always covered my scopes when they weren’t outside. Apparently the ceiling was wood but most wooden ceilings are vaulted which would make this scenario even less likely. But, I’m just an old, skeptical, retired fire officer who heard a lot of BS stories over the years. I could be overlooking an amazing collection of coincidences. I’d be checking their insurance and mortgage, among other things.

(a) I used to own a 10-12″ fresnel lens from Edmund Scientific way back when. I remember burning holes in 3/4″ plywood with it (I was reasonably patient…).

(b) A friend used to perform (juggling) with a 2+1/2″ crystal ball. Performing outdoors in Colorado on a sunny day once, he failed to close the cardboard set of drawers he kept it in. It started smoking prodigiously, fortunately it did not actually set his props on fire before he got to it, but it partially melted and scarred the ball (acrylic) and would almost certainly have set his stuff on fire if left for another minute or two.

(c) If the sun’s image is 7mm across, then the power deposited there depends on the frontal area of the scope. I can’t view the video (and I don’t see the size of the scope mentioned here), but even a 4″ scope (SCTs are usually much larger, I believe) would be concentrating the sun’s rays by a factor of about 1,000, putting several watts into the spot. Anything decently non-heat-conducting at the focus will get quite hot. It’s easily enough to ignite paper, even white paper.

(d) The irony thing might also have applied if this had happened in Surprise, AZ, or Sunrise, AZ.

(e) I wonder what would happen to the scope if it were just pointed at the sun. I suspect parts of the scope would get quite hot, and the scope might well eventually ignite.

Meade and Celestron recommend against using their SCTs for projection because the (still unfocused) image from the primary can heat up the secondary enough to damage it. Could the secondary have heated up, melted its plastic holder, and somehow started the fire? The giant hole in the corrector may have been caused by the heat of the secondary, not falling debris.

That said, I’d really like to see MythBusters take this one on. They had a great time with Christmas tree lights a while ago…

And another personal experience with unexpected lenses: I was eating lunch outdoors once, and the sun shining through my water bottle melted the nylon webbing on my backpack. I was sitting with a bunch of fellow astronomy grad students, so geeky discussion naturally ensued.

I don’t know…the way that it is explained seems like it could be plausible. I’ve never seen ANY projection from my eyepiece onto the ceiling, from any light source (street lights, flashlights, etc), though.

I think this weekend I may perform an experiment involving my 8″ SCT (covered), my Galileoscope (mounted as a finder scope), a mirror and diagonal, and an eyepiece with paper rubber-banded to it. I wonder which EP would be best…with the following eyepieces, I get these powers of magnification:

I have a Barlow that I can use to double that. Since sharpness decreases as power increases, would it be best to use something like a 25mm for this, or a 40mm? The 6mm wouldn’ t be as sharp, and thus would appear “dim” (in relative terms to sunlight, of course), correct?

@10 Calli Arcale: Fire can do very weird things sometimes, and casual experiments like trying to light paper with a Newtonian are probably not going to give you absolute answers to what would happen in this specific situation.

This. As an obsessive fan of Mythbusters, I can recall several episodes where they tried to ignite flammable vapors in an enclosed space with a spark and were repeatedly frustrated because they couldn’t get the fuel/air mixture just right. However, if I’m in a room full of gasoline fumes, I think I’m gonna put off flicking my Bic, all the same!

Come to think of it, a lesson I take from Mythbusters is that sometimes anything that can go wrong will go wrong, even if you’re trying to “make things go wrong” yourself (in which case nothing will go wrong). The episode where they spent about an hour unsuccessfully trying to get a skunk to spray them was a great example. I don’t think anyone denies the existence of skunk spray 😉

Let alone the (remote) possibility that a Schmidt-Cassegrain aiming at a random, but (I presume) fix point in the sky can cause an instant fire by focusing a moving Sun, it struck me that neither the story nor any of the comments mention the (equally legendary) account of the Archimedes’ burning-mirrors at the siege of Syracuse… Carefree or Historyfree?

@17 k: My friend’s car almost caught fire. We both have the same gear shift knob–it’s a dragon holding a glass orb. In his low-rider, the sun caught the orb and the seat was smoking when he noticed. In my truck, wrong angle, but we both use (matching) dragon sun shields now.

I seriously lol-ed. Sorry, that just strikes me as funny. Any problem caused by a dragon can be solved by a dragon!

1) If the sun is slightly off axis you can get a slightly out of focus area inside a scope near the tube for the eyepiece. I have seen Dobsonians start smoking from the partial reflection off the secondary onto the tube just next to the eyepiece. It’s easy to imagine this happening for other geometries in similar circumstances. This also gives you a wider area from which to start the first since we’re talking about pointing the scope “near” the sun, but not absolutely at it.

2) Frequently people hang things in a house or room (or even observatory) that could end up dangling near the focal point.

3) People at times do things that are a bit stupid. If you put the right optics in, you could extend the focal point significantly beyond the normal place where you position an eyepiece. There are a variety of reasons to increase the focal length of a particular scope. Perhaps someone was trying to project an image of the sun on the wall and messed up the calculation. Instead of a nice large image several inches (even a couple of feet) away they end up with a much smaller image formed on the wall(or ceiling).

I have a problem with the fire department saying that the place where the fire was the most intense was therefore the most likely place of origin. I can think of a few ways in which that might not be the case. What if there was some materials in that room that simply burned hotter for a longer period of time than the typical things found in a house? What if the fire started elsewhere in a material that was quick to ignite at low temperature and burn with less intensity than most housing material, like sawdust or cotton wool that might not leave much of a trace behind? What if he recently painted or lacquered the area near the ‘scope and the surface was still volatile? What if the shape of the room caused a micro-climate that concentrated the heat of the fire in certain areas despite having started elsewhere?

I have read several articles that criticize fire investigators for making many assumptions that simply aren’t based on tested scientific principles. I’m willing to accept their expert opinion on this for now, but I hope that they do consider other scenarios.

Having said all of that, it does seem conceivable that the telescope was involved based on its location and the lack of enclosure around it. Also, as mentioned earlier, checking out a possible electrical short if the ‘scope has a drive unit would be high on the list.

Phil – try it yourself (maybe with a flash light rather than the sun) – as has already been said, there are really three spots where an SCT could star a fire – at the eyepiece, at the secondary, or just in front of the optical tube if the sun is shining in at an angle.
With or without an EP, a spot several feet from the back of the scope would have a large image circle, and low intensity. Now a fire INSIDE the scope, that I can totally believe. I’ve seen it happen, as have several others in the comments.
Also, remember that eyewitness accounts are the least trustworthy.

Interesting… worst I did this week was put the cap over the eyepiece before I put the main lens cap on my 8″ SCT while viewing the sun for the transit… wife commented on how she didn’t think there a hole in the center of the eyepiece cap before. Whoops!

When I first got my Solar filter for my first generation ETX (first gen, that’s important), I lined it up on a window mount in my car and tried to get a look at the sun. I no sooner found the sun in the main scope then my head really hurt and I smelled something I shouldn’t have smelled.

The 35mm film canister I used for a cover of the straight through 6×22 finder with it’s 2mm eye relief (stupid design) fell off somehow and I did a crude form of laser hair removal.

Manufacturers say to never point any kind of Cassegrain (SCT, Mak, etc.) at the sun without the proper objective-end filter because internal parts could overheat, and, unlike a Newtonian, would have very limited ways to shed the heat. They warn that the lube that allows the focuser to move the primary mirror could all drip down to one side, and other parts could melt. @Tog: finderscopes might not be exempt!

Maybe we could get Meade, Orion, and Celestron to pitch in and talk Mythbusters into covering the topic.

We used to demonstrate the risks of solar observing by igniting pencils just behind the eyepiece of a 10-inch refractor on campus. Not all that good an idea with teenaged groups, as there’s always somebody who likes it and wants more details. It’s at least plausible for the telescope to have been involved if there was some source of ignition just behind the eyepiece. (no speculation on how likely that might have been).

For those who say a large image produces little heat, I disagree. I worked at the Buffalo Museum of Science and we used a heliostat to project the sun into our “sun show room”. The resulting image was 8″ image of the sun and your arm could get warm pointing out some of the sunspots.

That’s not necessarily true with an SCT. Unlike a Newt, the focal plane isn’t fixed, and can be moved by moving the mirror in and out. Some SCTs have a large range of focal plane movement. Nonetheless, it seems unlikely that the focal plane could be moved enough to actually hit the ceiling.

There’s a couple of possible scenarios here: one is that there was no eyepiece in the diagonal, which would require the focal plane to be be a long way out to cause this. This one doesn’t seem that likely.

But there’s another one: if there was an eyepiece in the scope, the projected image might not diverge a long way back from the eyepiece. With the focus set right, the rays emerging from the eyepiece would be close to parallel again and could in fact result in a hotspot a long way back from the scope. This scenario seems a lot more plausible…

@Richard
No, it happened at the East Coast Star Party, near Coinjock, NC and the scope involved was the star party’s coordinator, Kent Blackwell (often quoted by Sue French in her S&T articles) and it was his 25″ f/5 Obsession clone. Even the old pros goof up now and again =-)

We all got a good chuckle out of it at the time and still rib him about it.

As common as it is to see scopes using the sun to dry out the next morning, it is very possible you saw something on CN other than the event I witnessed.

I wonder how common it is for people to build purpose-built solar telescopes?
I’m a total n00b, but I’m thinking a purpose-built Newtonian (equatorial mount, of course) with an objective filter bracket made for heavy-duty filters (including improvised ones like big chunks of welding glass) and high-temp-tolerant materials throughout would work well. Maybe a shroud of wire mesh so that any stray internal reflections (of the internally damaging sort) will be visible right away to bystanders?

Manufacturers say to never point any kind of Cassegrain (SCT, Mak, etc.) at the sun without the proper objective-end filter because internal parts could overheat, and, unlike a Newtonian, would have very limited ways to shed the heat. They warn that the lube that allows the focuser to move the primary mirror could all drip down to one side, and other parts could melt. @Tog: finderscopes might not be exempt!

I accidentally left the cover off my finderscope while adjusting my Newtonian on Tuesday; even though my hand was only in the beam for a moment, and well away from the focal point, I could very much feel the heat. Interestingly, my Galileoscope was nowhere near as hot, and proved very good for projecting an image of the Sun. (I kept the filter over it the rest of the time, as a precaution not only against fires but against somebody, namely myself, going to look through it and forgetting.)

Duncan Kitchin:

But there’s another one: if there was an eyepiece in the scope, the projected image might not diverge a long way back from the eyepiece. With the focus set right, the rays emerging from the eyepiece would be close to parallel again and could in fact result in a hotspot a long way back from the scope. This scenario seems a lot more plausible…

Given that the individual apparently bought the ‘scope to start a new hobby (boy, I’d love to have enough spare cash to casually buy a 14″ SCT!), didn’t keep the cover on, and kept it set up on his patio for observing the night sky, I’d almost bet money that he kept an eyepiece in.

RapidEye:
No, it probably was the same incident but I just conflated it with the cedar tree in the observing field at Green Bank. The particulars, with the smoking cedar tree image sticks with me. Some Cloudy Nighter was relating the story to the crowd.

I’d like to attend one of the Coinjock events some time, Kent and other BBAAers rave about it! May’s event is past, though. Of course they’re stuck with severe light pollution in Va. Beach. We at CAS will have to have them up here for our astronomy retreat this summer. Like you, I’m on the BBAA Yahoo group, so I can invite them easily. We got clouded out in August & September last year. It ended up just me & one guy from NOVAC in October at Phil’s old stomping ground, Fan Mountain Observatory, home of the RRRT that you BBAAers have done such good work with.

Joseph:
Orion sells glass solar filters in various sizes that’d work well in your home-built scope. I’m not sure about welder’s glass filters when used with a telescope. There might still be a significant amount of IR or UV coming through that’ll cause eye trouble.

@Richard
If you get a chance, you’ve got to attend the ECSP – its always a great time, whether we get good weather or not!

Coinjock is far enough from VaB, the light pollution isn’t that bad – it takes out stuff low to the north, but above 30 degrees or so, its not a factor. And the views to the south and east, out over Currituck Sound are outstanding! At last month’s star party, we easily saw Omega Centauri and Centaurus A in binoculars and I could even glimpse Omega Centauri naked eye once I knew where it was in Binox.

And if you leave there hungry, its your own dang fault – lots of food, drinks, and fun! Spaghetti feast on Fri night, snacks all day long on Sat, and burgers and dogs on the grill Sat night. I just gained 5 pounds THINKING about last month’s star party =-)

You’ll also never run into a more laid back and fun bunch of people at a star party. Very kind and friendly to strangers/n00bs that come over from the campgrounds.

I know some of the BBAA guys that hit Coinjock also visit Green Bank – I’m quite certain the tale of Kent’s 25″ fire starter have traveled at least that far =-)

I’m doubtful of this. I myself, was using an 8″ SCT to watch the transit & take photos. While the sun was higher up, I definitely used a solar filter so I didn’t burn out my camera, or my eyes. But, within about a half hour of sunset, I could look through the scope without the filter. Yes, it was bright, but nowhere near the plasma cannon beam you’d get coming out of the eyepiece during mid day, and even that isn’t very focused after a few inches (speaking from experience). Sure, it’ll project a bright spot of illumination on a wall or ceiling, but it’s going to be spread out, unless you’ve got the scope focused so you can look through the eyepiece from several feet away.

Two points of bad astronomy: 1) They said “lenses” instead of “mirrors” (“Bunch o’ yokels–can’t tell a reflector telescope ain’t got no lenses.”), and 2) there would almost have to be two days of the year on which the sun would pass at the right position to start the fire.

Another point of skepticism: Discover had an article last year about fire science, and one of the important points was that the most intense burning is not necessarily where the fire started.

Re: “…the most intense burning is not necessarily where the fire started.”

Agreed, but let me point out something important. Such notions are still valuable when understood as rules of thumb, rather than absolute rules. They allow one to productively focus your attention and make rapid progress.

Often the goal is not to prove everything with mathematical certainty, before moving on to the next item and proving that the same way. Such methods can lead you to spend hours unproductively trying to move from 95% certainty to 100% certainty, and it’s all the more unhelpful when you have hundreds of possibilities to check out. You will rapidly find yourself bound by “analysis paralysis”.

Rules of thumb are used every day by Doctors, Technicians, Help Desks, and many others. The fact that they are not mathematically pure certainties detracts not at all from their value. They fail only when they are mistaken for what they are not.

Not just mirrors are a problem. I have one of those glass globe things that you fill with water and display a flower inside. It happened to be filled and sitting on a window sill, behind a closed curtain. I happened to come into the dining room and saw a tiny wisp of smoke from the curtain. The sunlight had been focused through the globe and had burnt a hole about 1/4in wide and an inch long in the curtain material. It is now no longer filled with water and/or a flower.

Yawn! Where’s the fun in that? 😛
Heh, actually I was just thinking about the fun of designing and building my own telescope. That’s one of the things in my bucket list, and it’s relatively doable, I understand, with the right research.
It’s cool there’s a place to go if you’re in the market, though 😉

@51 Richard: Orion sells glass solar filters in various sizes that’d work well in your home-built scope. I’m not sure about welder’s glass filters when used with a telescope. There might still be a significant amount of IR or UV coming through that’ll cause eye trouble.

Thanks, I’ll keep that in mind! Though welding arcs are famous for UV and I’d be willing to bet that welding glass blocks IR for the same reason. But yeah, it’s still probably much better to be safe then creative when it comes to that particular part

Oh, yes, this is possible! It is the reason the carpet (since replaced) at Fremont Peak Observatory had arched burn marks in it. Evidently as an operator (whose name shall forever remain secret) moved the 30″ Newtonian telescope about it came within the sun’s reach, and because there was an eyepiece on the floorward side of the telescope an intense beam of sunlight cut across the carpet! Several times, I might add

Watched a Galileoscope fitted with a Sunfunnel catch fire while watching the transit the other day at Lowell Observatory in Flagstaff AZ. The smoke escaped near the mounting lug, but it was unclear where the tube was actually burning inside. Demonstrates that not only could focussed rays ignite an external object, reflected rays (it was an SCT) could ignite an external object, but that internal heat (even with only internal mirrors) could have built up making the scope itself the initial ignition object.

#48 Joe G asks “I wonder how common it is for people to build purpose-built solar telescopes?”

Homebuilt solar telescopes are actually quite popular with ATMs. Here’s a common design for a Newtonian solar telescope utilizing uncoated mirrors and a Herschel Wedge: http://www.considine.net/dgroski/wlnewt/

Also, that should not be used as a rule of thumb. Where the fire is most intense is usually the place with the best ventilation. Using the belief that the most intense fire is the place where it started is like using a rule of thumb that most rivers flow south. It may be true in a particular case, but you can’t reasonably depend on it–nowhere near 95%.

I have installed a sprinkler system on my Meade ETX-125 and wear fire-protective gear at all times. Just in case, I have an ABC fire extiguisher affixed to my tripod. No ‘scope is g0ing to burn down my house.

Funny story to add here…I had rigged up my solar filter on one of my 7×35 binocular eyepieces. I put electrical tape over the other eyepiece so I wouldn’t look in by accident. Fortunately, I put the filter on the left eyepiece and looked into it with my right eye, leaving the right eyepiece (covered with tape) pointing over my shoulder. Watched the transit for only a few seconds and pulled back to look at the bonics…there was a smoldering hole in the tape over the right eyepiece. Good thing I didn’t stick my other eye there! I added a lot more tape and stern warnings (plus supervision) when I let other people use my rig. What I should’ve done was put a cover on the lens instead of trying to stop the light after it was already magnified and focused – d’oh!

A similar thing happened to my family when I was a little kid; about thirty years ago, my mother saved up for an 8″ Meade scope. Unfortunately, she didn’t have the chance to use it much. It was set up next to the living room window and only a couple weeks after purchase obligingly set the livingroom on fire. Luckily, noone was hurt, and the damage only took a couple weeks to repair. Amazingly, the telescope did survive and does still work — the main tube, one of the legs, and the houding for the eyepiece optics had to be replaced. It’s become kind of a family artifact, really.

I had a mobile rv repair business for a couple of decades and used a Chevy Astro van to haul my tools around.

I kept the toolbox at the rear doors of the van. These doors were open as I returned to the van to grab more tools I noticed a wisp of smoke rising from an open toolbox drawer.

Sunlight reflecting off the concave inside glass found a focal point on the rubber handle of a screw driver, and as I picked it up to show the customer standing beside me, the rubber mat in the bottom of the drawer started to smolder almost right away.

I wonder how many of these vans were used for deliveries and how many cardboard boxes have been set alight!

I’m surprised that so many people are skeptical of this. When I was in graduate school, some fellow TAs and I were aligning the 12-14″ Schmidt-Cassegrain that the department had for student use to do some solar projection stuff for some students. After a moment of moving the telescope around we smelled something burning and realized we had accidentally set one of us (his jacket, to be specific) on fire! We put the objective filter on then until we had the alignment set up properly. We had not done any focusing of the eyepiece, obviously, since we were not looking through the telescope (needless to say). I have no doubt that this very well could have happened, although, of course, “could have” != “did”.